1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming system, including an image forming unit for forming an image on one or both sides of a sheet.
2. Description of Related Art
In image forming apparatuses for forming images on sheets, such as printers, copy machines or multi function printers, some image forming apparatus can restrict printing or copying of highly confidential or important documents. For example, an image forming apparatus which prohibits operations such as copying the front side of a sheet if a specific pattern is printed on the back side thereof has been put into practical use (see, for example, Japanese Patent Application Laid-Open No. 2000-307857). For example, by transmitting an instruction (hereinafter referred to as a confidential document instruction) indicating the data of a confidential document or a confidential document from a personal computer connected to an image forming apparatus, a specific pattern as mentioned above is printed on the back side of the sheet on which the confidential document is printed.
Only when a special operation such as transmission of a confidential document instruction is performed in printing a highly confidential or important document (hereinafter referred to as an important document), a specific pattern as mentioned above is formed on the back side of a sheet. Hence, a specific pattern is not necessarily always printed on the back side of an important document. Moreover, as the important documents, there are documents created within a company and documents received from outside the company. There is a low possibility that important documents received from outside the company has a specific pattern printed on the back side thereof, and a specific pattern is not necessarily always printed on the back side of important documents created within the company. Thus, there is a high possibility that the back side of many of important documents is blank.
In general, paper (PPC paper) is used as sheets. Therefore, for the purpose of effectively using resources, it is recommended to use the back side of sheets printed only on one side. However, in terms of security, it is of course not preferred to use the back side of important documents as paper for printing other document or as memo paper. However, we cannot deny the possibility that the back side of sheets on which an important document is printed may be used by mistake as memo paper, or printing paper for other document.